This project is concerned with the study of muscle biochemistry using cultured beating rat heart cells as the tool. The main objectives of this work are (1) to determine the involvement of cyclic nucleotides in the inotropic and chromotropic responses of the cultured rat heart cells and to relate changes in the rate and intensity of beating with the age of the cells in culture, (2) to determine the role of cortisol in the reinitiation of beating in cells which have ceased beating because of advanced age, and (3) to determine the relationships between lipid metabolism and energy production for beating. Some recent work concerned with energy metabolism has involved a study of the effects of anesthetics on beating, ATP turnover, ATP concentrations, and creatine phosphate concentrations. It may be possible to use the beating heart cells to help explain the mechanisms of anesthesia. The effects of cortisol on myosin biosynthesis is being studied at the present time.